1. Field of the Invention
The present invention concerns methods and systems for acquiring magnetic resonance data from an examination subject (patient) wherein the patient is radiated with radio-frequency (RF) energy that subjects the patient to RF heating.
2. Description of the Prior Art
Radiofrequency (RF) heating at the tip of metallic wires is a significant safety problem in MRI (magnetic resonance imaging). Electric field in MRI induces currents on metallic wires which flow through the body and cause SAR (specific absorption rate) amplification near the wire tip.
Previous studies in the literature (Baker et al. “Evaluation Of Specific Absorption Rate as a Dosimeter of MRI-related Implant Heating.” Journal of Magnetic Resonance Imaging 20(2) (2004) pgs 315-320 and Vahihaus et al. “MR Imaging and Cardiac Pacemakers: In Vitro Evaluation and in Vivo Studies in 51 Patients at 0.5 T.” Radiology 215(3) (2004) pgs 869-879) assessed this heating for both in vitro and in vivo experiments. In other studies more theoretical approaches were governed to analyze the heating problem of implant leads by using mathematical models (Yeung et al. “A Green's function approach to local rf heating in interventional MRI.” Medical Physics 28(5) (2001) pgs 826-832 and Yeung et al. “RF Safety of Wires in Interventional MRI: Using A Safety Index.” Magnetic Resonance in Medicine 47(1): (2002) pgs 187-193. The validity of these models is then verified by comparison to experimental data. A detailed analysis of the problem was made in Yeung et al. above by solving the bio-heat equation with Green's function approach and linear system theory. Maximum steady state temperature increase in a tissue near a transmitter catheter antenna was calculated. In (see Yeung et al. above) a parameter named “safety index” which combines the effect of SAR gain of the implant lead and the bio-heat transfer process was presented. The variation of safety index with respect to implant lead length and radius, insulation thickness, tissue conductivity and permittivity was also investigated. These studies presented a good model of the tissue heating problem due to metallic wires in RF fields.
The modification of the implant leads and wires for RF heating reduction was investigated in other studies. In two of these studies a series of chokes was added to coaxial cables (Mark E. Ladd, H. H. Q. “Reduction of Resonant RF Heating in Intravascular Catheters Using Coaxial Chokes.” Magnetic Resonance in Medicine 43(4): (2000) pgs 615-619. and Ferhanoglu et al. “MRI Compatible Pacemaker Leads”, ISMRM 2005 p 963). Amplitude of the currents induced on the cable shield was reduced with this method. In another work Gray et al. “Simple Design Changes to Wires to Substantially Reduce MRI-Induced Heating at 1.5 T: Implications for Implanted Leads.” Magnetic Resonance Imaging 23(8): (2005) pgs 887-891) the effect of the coiled wires on the heating was investigated. Self resonance frequency of a coiled wire was shifted to the operating frequency by introducing air gaps and decreasing the parasitic capacitance. With this method it was possible to increase the impedance of the coiled wire and therefore reduce the RF heating. All of these designs are based on modifying the lead wires or cables. With these methods, however, it is difficult to produce mechanically robust leads. In addition, for the patients who already live with pacemakers, the exchange of leads with the modified safe ones may not always be feasible. Because of these reasons modification of implant lead designs or catheters may not always be the most appropriate solution to RF heating problem of metallic wires in MRI.
In a recent study (Nordbeck et al. “Spatial Distribution of RF-induced E-fields and Implant Heating in MRI” Magnetic Resonance in Medicine 60(2): (2008) pgs 312-319), the relationship between electric field distribution and the temperature rise of implant leads is investigated. It was found that orientation of the implant lead with respect to the direction of the electric field may result in different temperature increases however the approach of optimizing the EM transmitter field in order to minimize implant heating was not investigated. In order to find the worst case scenario, an optimization based approach was used in Yeung et al. “RF Heating Due to Conductive Wires During MRI Depends on the Phase Distribution of the Transmit Field” Magnetic Resonance in Medicine 48(6): (2002) pgs 1096-1098 to calculate the EM (electromagnetic) field which can generate maximum heating at the wire tip.